If more than one identity of a given
type is present in the certificate
(e.g., more than one dNSName name, a
match in any one of the set is
considered acceptable.) Names may
contain the wildcard character * which
is considered to match any single
domain name component or component
fragment. E.g., *.a.com matches
foo.a.com but not bar.foo.a.com.
f*.com matches foo.com but not
Internet Explorer behaves in the way outlined by the RFC, where each level needs its own wildcarded certificate. Firefox is happy with a single *.domain.com where * matches anything in front of domain.com, including other.levels.domain.com, but will also handle the *.*.domain.com types as well.
So, to answer your question: it is possible, and supported by browsers.